Transport Layer: Part I

Transcription

1 Transport Layer: Part I Transport Layer Services connection-oriented vs. connectionless multiplexing and demultiplexing UDP: Connectionless Unreliable Service TCP: Connection-Oriented Reliable Service connection management: set-up and tear down reliable data transfer protocols (Part II) flow and congestion control (Part II) Readings: Chapter 3, Lecture Notes CSci4211: Transport Layer:Part I 1

2 Transport Services and Protocols provide logical communication between app processes running on different hosts transport protocols run in end systems send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer more than one transport protocol available to apps application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical Internet: TCP and UDP CSci4211: Transport Layer:Part I 2

3 Transport vs. Application and Network Layer application layer: application processes and message exchange network layer: logical communication between hosts transport layer: logical communication support for app processes relies on, enhances, network layer services Household analogy: 12 kids sending letters to 12 kids processes = kids app messages = letters in envelopes hosts = houses transport protocol = Ann and Bill network-layer protocol = postal service CSci4211: Transport Layer:Part I 3

4 End to End Issues Transport services built on top of (potentially) unreliable network service packets can be corrupted or lost Packets can be delayed or arrive out of order Do we detect and/or recover errors for apps? Error Control & Reliable Data Transfer Do we provide in-order delivery of packets? Connection Management & Reliable Data Transfer Potentially different capacity at destination, and potentially different network capacity Flow and Congestion Control CSci4211: Transport Layer:Part I 4

5 Internet Transport Protocols TCP service: connection-oriented: setup required between client, server reliable transport between sender and receiver flow control: sender won t overwhelm receiver congestion control: throttle sender when network overloaded UDP service: unreliable data transfer between sender and receiver does not provide: connection setup, reliability, flow control, congestion control Both provide logical communication between app processes running on different hosts! CSci4211: Transport Layer:Part I 5

6 Multiplexing/Demultiplexing Demultiplexing at rcv host: delivering received segments to correct application process = API ( socket ) = process Multiplexing at send host: gathering data from multiple app processes, enveloping data with header (later used for demultiplexing) application P3 P1 P1 application P2 P4 application transport transport transport network network network link link link physical physical host 1 host 2 host 3 physical CSci4211: Transport Layer:Part I 6

7 How De-multiplexing Works host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries 1 transport-layer segment each segment has source, destination port number (recall: well-known port numbers for specific applications) host uses IP addresses & port numbers to direct segment to appropriate app process (identified by socket ) 32 bits source port # dest port # other header fields application data (message) TCP/UDP segment format CSci4211: Transport Layer:Part I 7

8 UDP: User Datagram Protocol [RFC 768] no frills, bare bones Internet transport protocol best effort service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of others Why is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desired CSci4211: Transport Layer:Part I 8

9 UDP (cont d) often used for streaming multimedia apps loss tolerant rate sensitive other UDP uses DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error recovery! Length, in bytes of UDP segment, including header 32 bits source port # dest port # length Application data (message) checksum UDP segment format CSci4211: Transport Layer:Part I 9

10 UDP Checksum Goal: detect errors (e.g., flipped bits) in transmitted segment Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1 s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? More later. CSci4211: Transport Layer:Part I 10

11 Checksum: Example (from book) arrange data segment in sequences of 16-bit words sum: checksum(1 s complement): binary addition, with overflow wrapped around verify by adding: CSci4211: Transport Layer:Part I 11

12 TCP: Overview point-to-point: one sender, one receiver reliable, in-order byte steam: no message boundaries pipelined: TCP congestion and flow control set window size send & receive buffers socket door application writes data TCP send buffer segment application reads data TCP receive buffer socket door full duplex data: bi-directional data flow in same connection MSS: maximum segment size connection-oriented: handshaking (exchange of control msgs) init s sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver CSci4211: Transport Layer:Part I 12

13 URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) TCP Segment Structure 32 bits source port # dest port # head len sequence number acknowledgement number not used UAP R S F checksum rcvr window size ptr urgent data Options (variable length) application data (variable length) counting by bytes of data (not segments!) # bytes rcvr willing to accept CSci4211: Transport Layer:Part I 13

14 TCP Seq. # s and ACKs Seq. # s: byte stream number of first byte in segment s data ACKs: seq # of next byte expected from other side User types C host ACKs receipt of echoed C Host A red: A-to-B Host B green: B-to-A simple telnet scenario host ACKs receipt of C, echoes back C time CSci4211: Transport Layer:Part I 14

15 timeout TCP: Error Scenarios Questions for you: How to detect lost packets? How to recover lost packets? Potential consequence of retransmission? How to detect duplicate packets? State maintained at sender & receiver? time Host A Host B X loss lost data scenario CSci4211: Transport Layer:Part I 15

16 timeout timeout TCP: Error Scenarios (cont d) Host A Host B Host A Host B X loss time time lost ACK scenario duplicate packets CSci4211: Transport Layer:Part I 16

17 A Simple Reliable Data Transfer Protocol Stop & Wait Protocol (aka Alternate Bit Protocol) Sender algorithm: Send Phase: send data segment (n bytes) w/ seq=x, buffer data segment, set timer Wait Phase: wait for ack from receiver w/ ack= x+n if received ack w/ ack=x+n, set x:=x+n, and go to sending phase with next data segment if time out, resend data segment w/ seq=x. if received ack w/ ack!= x+n, ignore (or resend data segment w/ seq=x) Receiver algorithm:?? CSci4211: Transport Layer:Part I 17

18 SRDTP: Finite State Machine : state Sender FSM event action : transition Receiver FSM??? Upper layer: send data (n bytes) make data sgt, seq = x, set timer pass data sgt to lower layer Send phase Wait phase receive Ack w/ ack = x+n x: = x+n, stop timer receive Ack w/ ack!= x+n no op, or resend data sgt time out resend data sgt info ( state ) maintained at sender: phase it is in (send, or wait), ack expected, data sgt sent (seq #), timer CSci4211: Transport Layer:Part I 18

19 TCP Connection Set Up Three way handshake: TCP sender, receiver establish connection before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info client: end host that initiates connection server: end host contacted by client Step 1: client sends TCP SYN control segment to server specifies initial seq # Step 2: server receives SYN, replies with SYN+ACK control segment ACKs received SYN specifies server receiver initial seq. # Step 3:client receives SYN+ACK, replies with ACK segment (which may contain 1 st data segment) CSci4211: Transport Layer:Part I 19

20 TCP 3-Way Hand-Shake initiate connection client server SYN received Question: a. What kind of state client and server need to maintain? connection established connection established b. What initial sequence # should client (and server) use? CSci4211: Transport Layer:Part I 20

21 3-Way Handshake: Finite State Machine Client FSM? Server FSM? Upper layer: initiate connection sent SYN w/ initial seq =x???? closed?? SYN sent conn estab ed?? SYN+ACK received sent ACK info ( state ) maintained at client? CSci4211: Transport Layer:Part I 21

22 Connection Setup Error Scenarios Lost (control) packets What happen if SYN lost? client vs. server actions What happen if SYN+ACK lost? client vs. server actions What happen if ACK lost? client vs. server actions Duplicate (control) packets What does server do if duplicate SYN received? What does client do if duplicate SYN+ACK received? What does server do if duplicate ACK received? CSci4211: Transport Layer:Part I 22

23 Connection Setup Error Scenarios (cont d) Importance of (unique) initial seq. no.? When receiving SYN, how does server know it s a new connection request? When receiving SYN+ACK, how does client know it s a legitimate, i.e., a response to its SYN request? Dealing with old duplicate (aka ghost ) packets from old connections (or from malicious users) If not careful: TCP Hijacking How to choose unique initial seq. no.? randomly choose a number (and add to last syn# used) Other security concern: SYN Flood -- denial-of-service attack CSci4211: Transport Layer:Part I 23

24 TCP: Closing Connection Remember TCP duplex connection! Client wants to close connection: Step 1: client end system sends TCP FIN control segment to server Step 2: server receives FIN, replies with ACK. half closed Step 3: client receives FIN. half closed, wait for server to close Server finishes sending data, also ready to close: Step 4: server sends FIN. client closing half closed client server half closed server closing CSci4211: Transport Layer:Part I 24

25 TCP: Closing Connection (cont d) Step 5: client receives FIN, replies with ACK. connection fully closed Step 6: server, receives ACK. connection fully closed Well Done! Problem Solved? client closing half closed full closed client server half closed server closing full closed CSci4211: Transport Layer:Part I 25

26 Two-Army Problem CSci4211: Transport Layer:Part I 26

27 timed wait TCP: Closing Connection (revised) Two Army Problem! Step 5: client receives FIN, replies with ACK. client closing client server half closed Enters timed wait - will respond with ACK to received FINs half closed server closing Step 6: server, receives ACK. connection fully closed X timeout Step 7: client, timer expires, connection fully closed full closed full closed CSci4211: Transport Layer:Part I 27

28 TCP Connection Management FSM TCP client lifecycle TCP client lifecycle CSci4211: Transport Layer:Part I 28

29 TCP Connection Management FSM TCP server lifecycle TCP server lifecycle CSci4211: Transport Layer:Part I 29

30 Socket: Conceptual View socket() CSci4211: Transport Layer:Part I 30

31 BSD Socket Programming (connectionless) CSci4211: Transport Layer:Part I 31

32 BSD Socket Programming Flows (connection-oriented) CSci4211: Transport Layer:Part I 32

33 Transport Layer: Part I Summary Transport Layer Services Issues to address Multiplexing and Demultiplexing Connectionless vs. Connection-Oriented UDP: Unreliable, Connectionless TCP: Reliable, Connection-Oriented Packet ( Segment ) Format: Sequence #, ACK, flags, A Simple Reliable Data Transfer Protocol Connection Management: 3-way handshake, closing connection Preview of Part II: more efficient reliable data transfer protocols round-trip time estimation and flow/congestion control CSci4211: Transport Layer:Part I 33